1. Field of the Invention
The present invention relates to an ultrasonic motor for driving a rotor by a travelling vibration wave generated in an elastic member by utilizing a vibration generated when a high frequency voltage is applied to a piezoelectric member of a stator.
2. Related Background Art
In prior art ultrasonic motors, two piezoelectric members are bonded to form a stator (Japanese Laid-Open Patent Application 60-91878), or two sets of electrodes are radially arranged (Japanese Laid-Open Patent Application 60-190178) in order to attain size reduction, easy forward/reverse rotation, high durability and high response.
However, where the two piezoelectric members are bonded together, a drive load is different between one member and the other member and a control therefor is difficult. Where the two sets of electrodes are radially arranged, if the dimensions of segments are different between an inner periphery and an outer periphery, wavelengths of the travelling vibration waves are different and a drive efficiency is lowered. Further, if the divisions of the segments of the electrodes on the inner periphery and the outer periphery are equal, the numbers of waves of vibrations generated on the inner periphery and the outer periphery are different and the control is difficult.
In any of the above cases, it is difficult to detect the precise status of operation by a single monitor electrode. Thus, full power drive is effected even if a low power drive is sufficient, and the suppression of power consumption is hard to attain.